Dress appropriately for safety and security. Though rare, airline accidents do happen. Should it become necessary to evacuate the plane by using that inflatable slide, or worse, walking out onto the wing, the last things you want to be wearing are shorts, a tank top or spike heels. Keep it simple: cotton or other natural fiber clothing (synthetics are more flammable), long sleeves and pants, sensible shoes, preferably sneakers. If you don't wear sneakers, wear sturdy, comfortable shoes that are easy to put on and remove for security. Slip-on athletic or walking shoes are a good choice.
Keep your footwear on at all times on the plane. The last thing you want is fumbling with them when an emergency arises.